The preparation of mixed oxide compositions of vanadium and phosphorus and the use of these as catalysts for the oxidation of hydrocarbons to maleic anhydride is known in the art. In U.S. Ser. No. B330,354 and U.S. Pat. No. 4,111,963 the importance of reducing the vanadium used in a vanadium/phosphorus/oxygen catalyst to the +4 oxidation state is described. Preferred is the use of concentrated hydrochloric acid as the reaction medium to bring about this reduction and preferred catalysts have a phosphorus to vanadium atom ratio of 1:2 to 2:1 and a porosity of at least 35%. In U.S. Pat. No. 3,864,280 the reduction of the vanadium in such a catalyst system to an average valence state of 3.9 to 4.6 is emphasized; the atomic ratio of phosphorus to vanadium is 0.9-1.8:1. Isobutyl alcohol is used as a solvent for the catalyst preparation, with the indication that an increase in catalyst surface area, over that obtained from use of an aqueous system, is achieved. The addition of promoters to the vanadium/phosphorus oxide catalyst compositions used for the oxidation of hydrocarbons to maleic anhydride is also disclosed in the art. Thus, in U.S. Pat. Nos. 4,062,873 and 4,064,070 are disclosed vanadium/phosphorus/silicon catalyst compositions made in an organic medium. In U.S. Pat. Nos. 4,132,670 and 4,187,235 are disclosed processes for preparing high surface area vanadium/phosphorus oxide catalysts. Anhydrous alcohols of 1-10 carbon atoms and 1-3 hydroxyl groups are used to reduce the vanadium to a valence of 4.0 to 4.6. Also disclosed are such catalysts containing up to 0.2 mol, per mol of vanadium, of a transition, alkali or alkaline earth metal, for example, tantalum, titanium, niobium, antimony, bismuth or chromium. U.S. Pat. Nos. 4,151,116 and 4,244,878 disclose vanadium/phosphorus catalysts having an element selected from magnesium, calcium, scandium, yttrium, lanthanum, cerium, uranium, chromium, manganese, iron, cobalt, nickel, copper, zinc, aluminum, gallium, indium, silicon, germanium, tin, antimony, bismuth and tellurium post-deposited on the surface. Optionally, the catalyst includes an integrally-incorporated promoting or activating element selected from titanium, zinc, hafnium, lithium, magnesium, calcium, iron, cobalt, nickel, copper, tin, bismuth, uranium, the rare earth metals, chromium, cadmium, and aluminum.
In any commercial process for the production of maleic anhydride from n-butane it is important that the yield of maleic anhydride be as high as possible, that is, the product of n-butane conversion and selectively to maleic anhydride should be maximized. Thus, in a process in which the off-gases following recovery of maleic anhydride are combusted, a high single-pass yield is of import. In a recycle-type of operation wherein unconverted n-butane is returned to the oxidizer following removal of maleic anhydride, a high single-pass yield is also desirable in order to avoid reprocessing excessive amounts of hydrocarbon.
Albeit the fact that the art discloses the use of promoters in the improved vanadium/phosphorus oxide catalyst systems used in the oxidation of hydrocarbons to maleic anhydride, there remains a need for improved processes for maleic anhydride production. Such is an object of this invention. Another object is to provide an improved vanadium/phosphorus oxide catalyst system. Still other objects will become apparent hereinafter.